Abstract

Adolescence is a critical developmental period during which most adult smokers initiate their habit. Adolescents are more vulnerable than adults to nicotine's long-term effects on addictive and cognitive behavior. We investigated whether adolescent nicotine exposure in rats modifies expression of nicotinic acetylcholine receptors (nAChRs) in medial prefrontal cortex (mPFC) in the short and/or long term, and whether this has functional consequences. Using receptor binding studies followed by immunoprecipitation of nAChR subunits, we showed that adolescent nicotine exposure, as compared with saline, caused an increase in mPFC nAChRs containing α4 or β2 subunits (24 and 18%, respectively) 24 h after the last injection. Nicotine exposure in adulthood had no such effect. This increase was transient and was not observed 5 wk following either adolescent or adult nicotine exposure. In line with increased nAChRs expression 1 d after adolescent nicotine exposure, we observed a 34% increase in amplitude of nicotine-induced spontaneous inhibitory postsynaptic currents in layer II/III mPFC pyramidal neurons. These effects were transient and specific, and observed only acutely after adolescent nicotine exposure, but not after 5 wk, and no changes were observed in adult-exposed animals. The acute nicotine-induced increase in α4β2-containing receptors in adolescents interferes with the normal developmental decrease (37%) of these receptors from early adolescence (postnatal day 34) to adulthood (postnatal day 104) in the mPFC. Together, this suggests that these receptors play a role in mediating the acute rewarding effects of nicotine and may underlie the increased sensitivity of adolescents to nicotine.

Plasma nicotine and cotinine levels during and after nicotine exposure. Plasma nicotine (A) and cotinine levels (B) in animals (n=8) exposed during adolescence (gray) or adulthood (black). Blood was collected 30 min after the first injection (N1.1), 30 min after the third injection on the same day (N1.3), and on the first day of withdrawal in the morning (W1). *P < 0.05, **P < 0.01 vs. adult animals.

Nicotine affects sIPSC frequency and amplitude in layer II/III pyramidal neurons differently depending on age of nicotine exposure. A) During whole-cell recording, layer II/III pyramidal neurons in mPFC were filled with biocytin for post hoc morphological identification. B) Example of whole-cell recordings during baseline, nicotine application, and washout using animals that were exposed to saline during adolescence. C, D) sIPSC frequency (C) and amplitude (D) are increased by nicotine (10 μM), and these effects are abolished in TTX (1 μM); gray rectangles highlight the time points taken for calculation of the nicotine effect (insets). n = 14 cells from 7 animals, TTX data n = 7 cells from 3 animals. E) Left panel: effect of nicotine (10 μM) application on sIPSC amplitude during the whole-cell recordings in animals 1–2 d after adolescent nicotine or saline exposure (same as in D). Right panel: cumulative distributions of sIPSC amplitudes at the peak of the nicotine effect (at time points highlighted in gray in left panel). F) Left graph: nicotine effect on sIPSC amplitude is larger in animals 1–2 d after adolescent nicotine exposure (average taken during time points highlighted in gray). Right graph: nicotine effect on sIPSC frequency is not altered 1–2 d after nicotine exposure (average taken at same time points as in left panel). Data are from n=6 animals (17–21 cells)/treatment group. *P < 0.05.